This invention relates to semiconductor lasers, and, more particularly, to an improved quantum well semiconductor laser.
Lasers are devices that convert energy of one type to another type by quantum resonance phenomena. Typically, noncoherent radiation having a range of wavelengths or injected charge carriers are used to pump charge carriers to excited energy states, which then decay or combine with other charge carriers, resulting in the emission of coherent radiation having a narrow wavelength range.
Lasers have become important tools of science and industry, because they can produce very intense beams of energy, and because the energy has only a narrow wavelength range. Many devices use lasers for one or a combination of these reasons. Lasers are used in communications systems, metalworking machines, surgical instruments, and electronic semiconductor devices, to name a few applications.
One increasingly important type of laser is the semiconductor laser, whose operation is based upon the excitation and recombination of charge carriers in a semiconducting material. Such semiconducting lasers are typically quite small, and are used primarily in electronic applications. One improvement to the basic semiconductor laser is the quantum well laser, wherein a thin layer of a semiconductor lasing material, the quantum well, is sandwiched between layers of another semiconductor that serve as the sources of charge carriers that are injected into the quantum well. The dimensions of such quantum wells can be very small, on the order of about 10.sup.-8 meters in thickness. Accordingly, their primary application is found in opto-electronics and scientific investigations. Other types of quantum well lasers employ multiple alternating semiconductor interlayers in the quantum well, in a structure termed a multiple quantum well or superlattice, increasing the output power of the laser and permitting other variations and features.
One type of quantum well laser of particular interest for use in electronic devices is based upon gallium arsenide and its alloys. In the simplest form, this laser has a layer of GaAs, the quantum well, between two layers of AlGaAs material, the two outer layers having a heteroepitaxial relationship with the quantum well. Electrons are injected into the quantum well from one AlGaAs layer, and holes are injected into the quantum well from the other AlGaAs layer. The AlGaAs layers are of a chemical composition, such as Al.sub.0.3 Ga.sub.0.7 As, that have a band gap larger than GaAs in order to confine the charge carriers within the quantum well. If the charge carriers were not localized within the quantum well, the laser would not be as efficient and would lase at longer wavelengths. The energies of the charge carriers in the AlGaAs layers are determined by the composition of the layer. Since the band gap of AlGaAs increases with aluminum concentration, the chemical compositions of the AlGaAs layers required to maintain the resonant confinement result in energies of the electrons within the AlGaAs layers that are typically much higher than the respective energy levels in the quantum well.
In a conventional quantum well laser, charge carriers injected into the quantum well region must lose energy to achieve the energy level for lasing. If the charge carriers cannot lose energy quickly enough, they are not captured within the quantum well and are lost to the surrounding environment. The efficiency and power output of the quantum well laser are then considerably less than might be otherwise attained. This limitation on the efficiency of capturing charge carriers is particularly acute for very thin quantum wells having thicknesses of 100 Angstroms or less, and for the electron charge carriers, which have longer scattering lengths than do hole charge carriers. Both considerations decrease the time available for the energy loss of the electrons to the lasing energy of the well, thereby decreasing the efficiency of capture.
Thus, there is a need for a technique and structure for increasing the efficiency and output power of quantum well lasers, and decreasing their threshold current, particularly by increasing the capture or collection efficiency of the charge carriers in the quantum well. The technique should be compatible with the fundamental quantum well laser process and structure. Desirably, it would be applicable to a wide variety of quantum well laser constructions, but in any event allow improved operation of the gallium arsenide class of quantum well lasers. The present invention fulfills this need, and further provides related advantages.